1. Field of the Invention
This invention relates to electrochemical cells. More specifically, the invention relates to electrochemical cells suitable for the detection and measurement of concentration of analytes in liquid samples.
2. Discussion of the Art
For conventional electrochemical analysis of a liquid sample, electrodes are dipped in the sample for electrochemical determination of the type of analyte or measurement of the concentration of analyte or both. The electrodes are spaced apart from each other, and the electrolytes in the sample provide ionic communication between the electrodes. In a majority of situations, the sample is static during measurement; in some instances, the sample flows through an electrochemical detector when the sample is in a fluid motion, such as in the case of flow injection analysis. The dimensions of the electrodes define the volume of the sample required for the measurement. The constraints relating to the volume of the sample and the requirement of rapid measurement may call for the use of microelectrodes, when the volume of the sample is not sufficient to cover the surface area of electrodes of conventional size.
Different methods of forming microelectrodes for the fabrication of electrochemical cells have been demonstrated. Interdigitated electrodes or band electrodes can be formed, with the electrodes being in close proximity to minimize the volume of sample required to perform an electrochemical measurement. In these devices, the electrodes are positioned on the same surface. U.S. Pat. No. 5,045,828 describes a humidity sensor comprising (a) a substrate having an electrically insulating surface; (b) a pair of spaced electrodes on the surface; and (c) a film having a thickness of approximately 5 microns or less on the surface interconnecting the electrodes. Conventional biosensors have a working electrode and a dual-purpose reference/counter electrode on the same major surface of an insulating substrate. The reactive chemistry is positioned on either the working electrode or on both the dual-purpose reference/counter electrode and the working electrode. U.S. Pat. No. 5,509,410 describes a sensor system adapted for releasable attachment to signal readout circuitry. The strip comprises an elongated support adapted for releasable attachment to readout circuitry; a first conductor and a second conductor each extending along the support and comprising means for connection to the circuitry. An active electrode, positioned to contact a liquid mixture and the first conductor, comprises a deposit of an enzyme capable of catalyzing a reaction involving the compound and preferably an electron mediator, capable of transferring electrons between the enzyme-catalyzed reaction and the first conductor. A reference electrode is positioned to contact the mixture and the second conductor. The system includes circuitry adapted to provide an electrical signal representative of the current.
WO 03/05639 discloses a microelectrode in the form of a receptacle. The receptacle comprises a working electrode in the wall of the receptacle, typically having a small surface area. A counter electrode is also present, the electrode typically having a much larger surface area than that of the working electrode, generally a surface area which is at least an order of magnitude larger than that of the working electrode. The electro-active substance may be placed into the receptacle and is optionally dried into position. The sample is then applied to the receptacle in order that testing can be carried out. The electro-active substance will typically not contact the working electrode in the wall of the receptacle during storage and therefore fouling of this electrode is minimized.
Various references in the prior art describe methods of fabrication of electrochemical cells for various analytical applications. Some of these references describe electrochemical cells having electrodes positioned side-by-side and having reagents on the surfaces of the electrodes, while other of the references describe electrochemical cells having a receptacle having one of the electrodes along the wall of an electrochemical cell and reagents positioned away from the active electrode. The positions and dimensions of the electrodes constituting the cell determine the volume of the electrochemical cell. Therefore, it would be desirable to provide electrochemical cells where the electrodes are positioned in such a manner as to decrease the volume of liquid sample required by the cell, in which positioning of reagents can be in contact with the working electrode.